Acidizing industrial equipment



United States Patent ACIDIZING INDUSTRIAL EQUIPMENT No Drawing. Application September 28, 1954 Serial No. 458,969

3 Claims. (Cl. 117-49) The invention relates to acidizing ferrous metal equipment so as to remove scale or rust such as may accumu-.

late in a steam generator or the like.

It more particularly concerns an improved method of removing acid-soluble scale or rust from a ferrous metal surface with an aqueousacid solution in which the tendency for the so-cleaned surface to become rusty while wet following the cleaning treatment is reduced or eliminated.

In the conventional acidization of industrial equipment, such as for example steam generaotrs, condensers, tanks, pipe lines, heat exchangers, and the like, the surface to be cleaned is treated with an inhibited acid solution. The usual mode of treatment is to fill the equipment with the acid solution if its volume in relation to the surface area to be treated is not excessive. Otherwise, the article to be cleaned may be dipped in the acid solution or the acid solution may be sprayed or squirted upon the surface to be cleaned. The acid is used in an amount and for a time sufiicient'to dissolve or dislodge the deposits to be removed. After the acidization, the acid treated surface is usually rinsed with water or an alkaline solution followed by water. The treatment thus leaves the surface clean but wet and in this condition exposure to the atmosphere causes undesirable rusting. Prompt drying of the cleaned equipment is not always feasible and does not entirely avoid rusting as considerable rusting inevitably occurs while eliminating moisture from the acid-cleaned surface.

It is the principal object of the invention to provide a method of acidization of a ferrous metal surface with one of the mineral acids, hydrochloric or sulfuric, whereby the tendency for the cleaned wet surface to rust before drying is greatly reduced if not entirely overcome. Other objects and advantages of the invention will appear as the description of the invention proceeds.

In carrying out the invention, the following sequence of steps is employed:

Step 1.The metal surface to be cleaned is subjected to the action of an aqueous solution of either hydrochloric or sulfuric acid, hydrochloric acid being preferred. Various concentrations of acid may be used. In the case of hydrochloric acid 5 to 30 percent HCl by weight may be used. A generally satisfactory concentration of HCl is about 7 percent, although other concentrations may be used. It is preferable to add a corrosion inhibitor to the acid solution, as understood in the pickling art, such as an organic nitrogen base, so as to protect the metal from attack by the acid without substantially affecting its action upon the deposits to be removed. The temperature at which the acid solution is applied to the Work is not critical, although faster action is had at elevated temperatures, such as 150 F. On the other hand, corrosive attack upon the underlying metal is aggravated by elevating the working temperature so that a compromise between fast action on the deposits and corrosive attack upon the metal must be made. In general, operating at temperatures up to 100 F. is satisfactory with many of the commercially available corrosion inhibitors.

governed by the nature of the equipment to be cleaned. Industrial equipment such as a boiler, vessel, tank, pipe line, etc. which is to be cleaned on the inside, and capable of holding a charge of the acid solution, can usually be cleaned by filling the equipment and letting the acid remainjin contact with the deposits to be removed until they are dislodged of dissolved. Small objects may be immersed in a bath of the acid solution. Large surfaces, as those of a large tank, the hold of a ship, may be cleaned byspraying the acid solution onto the surfaces to be cleaned. The so-sprayed'solution may be collected, and, if not spent, may be 'resprayed upon the surfaces until more or less spent so thata relatively small volume of solution may be used.'- 7

Step 2.--After the depositsare removed by treatment with the acid solution, the' next step is to rinse the'treated surfaces with water so as to rinse off most of the acid solution. This operation is performed in similar manner to step 1 and preferably rapidly so as to incur a minimum of exposure of the treated surfaceto the atmosphere. Complete rinsing is not resorted to because the presence of some acid in the final rinsings is desirable as this tends to prevent the formation of rust deposits while rinsing takes place. The pH of the water collected from rinsing or of the final rinse preferably should not exceed 2. A pH of not over 1 is desirable;

Step 3.The third step is to rinsethe water-rinsed surface with an aqueous alkaline solution, such as a water solution of sodium, hydroxide The concentration of alkali need not exceed 20' percent by weight. A generally useful concentration isqab'out 2 percent, although concentrations as low as 0.1 percent may be used. Rinsing with the alkaline solution is continued until all the acid is neutralized as indicated bythe rinsings becoming alkaline, e. g. having a pH of froin- 7 to 10. The alkaline solution is applied in similar manner to that of step 1.

Step 4.-The alkali-rinsed surface is next treated with an alkaline sodium nitrite solution. The nitrite solution may be prepared by dissolving an alkali, such as sodium hydroxide, and a soluble nitrite, such as sodium nitrite in water. Preferable concentrations for the nitrite are 0.25 to 2 percent and those of the alkali 0.1 to 5 percent by weight. The solution is applied in similar manner to that of the liquids used in the previous steps. The nitrite solution is used in amount sufficient to thoroughly displace'the liquids already on the surface being treated and provide at least a film of the nitrite-containing solution on the metal surface.

Step 5 .-'Il1e alkaline sodium nitrite solution is drained from the surface, as treated in step 4.

In some instances, the so-drained equipment may be returned to service while still Wet; in others it may be desirable to dry the equipment before returning it to service.

By the foregoing method, mill scale and other acid-sol.- uble deposits are removed completely from ferrous metal surfaces which are thereby left clean, generally grey in color (after drying), and there is no after rusting during drying.

Example The following example is illustrative of the practice of the invention: The inside side wall of a cylindrical tank 36 feet in diameter and 20 feet high formed of rolled steel plates welded together and having a capacity of 144,000 gallons was cleaned in accordance with the invention. At the time the cleaning operation was begun about 20 percent of the inside wall surface area had a coating of mill scale and the remainder of the surface had a red coating comprising mainly ferrous hydroxide and ferric oxide. 500 gallons of 28 percent hydrochloric Patented June 10, 1958 acid solution containing a corrosion inhibitor was uniformly sprayed over the inside wall surfaces at 80 F., the spraying being continueQffor 35 minutes at the rate of 14 to gallons per minute. As spraying proceeded, the solution, which was directed against th e upperend of the tank, trickled down the inside and collected on the bottom and was drained out. The acid solution spray was followed directly with a similar spraying of 100 gal.- lons of water at G. P. M. The water spray rinsed off most of the acid. The water spray was followed by an alkali water rinse consisting of 900 gallons of a 4 percent solution of sodium hydroxide. The alkali water rinse was applied to the tank wall by spraying as in the prior steps, the spraying time being 75 minutes and the spray rate about 12 gallons per minute. At the conclusion of the alkali water spray, the spent solution which reached the bottom of the tank had a pH ofj'liO. The alkali water spray was followed directly by a spray of an alkali sodium nitrite solution in the amount of 300 gallons applied to the tank in similar manner to that of the acid solution. The alkaline sodium nitrite solution contained in solution 15 pounds of sodium nitrite and 1'5 pounds of sodium hydroxide. After draining this solution from the bottom of the tank, the tank bottom was further rinsed with an additional quantity (200 gallons) of the alkaline sodium nitrite solution of the same composition as that used on the tank sides. The tank was then allowed to dry by exposure to the atmosphere. After the tank was dry, visual inspection showed that it was 100 percent clean. All the mill scale and red iron oxide and. hydroxide originally present was gone. The surfac'e'of the metal was clean and had a grey color. There was no rusting following the treatment and drying.

Although the invention has been described more particularly in connection with the use of'certain compounds, it is to be understood that theinvention is not to be limited thereto as equivalents thereof may be used. For example, in place of sodium hydroxide used in the third and fourth steps, there may be used sodium carbonat sodium bicarbonate, trisodium phosphate, disodium phosphate, sodium tetraborate, sofrlium silicate (Na SiO and the corresponding potassium compounds. In place of the sodium nitrite, ammonium or potassium nitrite may be used.

We claim:

1. The method of removing acid-soluble deposits from a ferrous metal surface which comprises the following steps carried out in immediate succession: (1) applying to the ferrous metal surface an aqueous solution of :1 mineral acid selected from the group consisting of sulfuric and hydrochloric acid in an amount and for a time sufiicient to remove the deposits; (2) rinsing the so-treated ferrous metal surface so as to remove most of the acid solution therefrom, the rinsing being terminated before the pH of the rinse water exceeds 2; (3) treating the water rinsed surface with an aqueous alkaline solution so as to render the water-rinsed surface alkaline to a pH of from 7 to 10; (4) treating the so-treated surface with an aqueous alkaline solution containing a soluble nitrite so as to displace liquids already on the surface being treated and provide at least a film of the nitrite-com taining solution on the metal surface; and (5) allowing the so-treated surface to drain.

2. The method according to claim 1 in which the mineral acid is hydrochloric.

3. The method according to claim 2 followed by the step of drying the drained surface.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,296 Shoemaker Mar. 30, 1943 2,243,853 Caster June 3, 1941 2,318,606 Goebel May 11, 1943 2,428,364 Flager Oct. 7, 1947 

1. THE METHOD OF REMOVING ACID-SOLUBLE DEPOSITS FROM A FERROUS METAL SURFACE WHICH COMPRISES THE FOLLOWING STEPS CARRIED OUT IN IMMEDIATE SUCCESSION: (1) APPLYING TO THE FERROUS METAL SURFACE AN AQUEOUS SOLUTION OF A MINERAL ACID SELECTED FROM THE GROUP CONSISTING OF SULFURIC AND HYDROCHLORIC ACID IN AN AMOUNT AND FOR A TIME SUFFICIENT TO REMOVE THE DEPOSITS, (2) RINSING THE SO-TREATED FERROUS METAL SURFACE SO AS TO REMOVE MOST OF THE ACID SOLUTION THEREFROM, THE RINSING BEING TERMINATED BEFORE THE PH OF THE RINSE EXCEEDS 2; (3) TREATING THE WATER RINSED SURFACE WITH AN AQUEOUS ALKALINE SOLUTION SO AS TO RENDER THE WATER-RINSED SURFACE ALKALINE TO A PH OF FROM 7 TO 10; (4) TREATING THE SO-TREATED SURFACE WITH AN AQUEOUS ALKALINE SOLUTION CONTAINING A SOLUBLE NITRITE SO AS TO DISPLACE LIQUIDS ALREADY ON THE SURFACE BEING TREATED AND PROVIDE AT LEAST A FILM OF THE NITRITE-CONTAINING SOLUTION ON THE METAL SURFACE, AND (5) ALLOWING THE SO-TREATED SURFACE TO DRAIN. 